This invention concerns an improved process for removing hydrogen fluoride from glycolic acid.
Recently hydrogen fluoride has been found to be a surprisingly effective catalyst for glycolic acid production. For instance, U.S. Pat. No. 3,911,003, granted Oct. 7, 1975, describes a process for preparing glycolic acid by contacting formaldehyde, carbon monoxide and water in the presence of a hydrogen fluoride catalyst. For economy of process and for subsequent processing of the crude product, it is necessary to separate the hydrogen fluoride catalyst from the crude glycolic acid product. It is usually necessary to reduce the hydrogen fluoride content of the product to a few parts per million, or even to less than one part per million, before the product can be used. It would be expected that, as hydrogen fluoride is quite volatile, it would readily be removed by distillation or by stripping with inert gas at moderate temperatures. For example, U.S. Pat. No. 2,534,017 shows that hydrogen fluoride can readily be removed from aromatic aldehydes by vacuum distillation at 55.degree. C. to 85.degree. C.; and U.S. Pat. No. 3,962,343 shows rapid removal of hydrogen fluoride in a similar system by stripping with a hydrocarbon diluent or by decomposing in a film-evaporator at 80.degree. C. to 110.degree. C. It has been found, in agreement with this, that most of the hydrogen fluoride can be removed from the crude glycolic acid-hydrogen fluoride product formed by the process of U.S. Pat. No. 3,911,003 by distilling or stripping with gas at temperatures at or below 110.degree. C. However, when the concentration of hydrogen fluoride in crude glycolic acid has been reduced to about 4% by weight, i.e., about 40,000 ppm, further stripping with an inert gas or distillation at these temperatures does not significantly reduce the hydrogen fluoride concentration.
Accordingly, in order to obtain a glycolic acid product having less than about 4%, by weight, of hydrogen fluoride, an improved process is desirable. Hydrogen fluoride is an unwanted contaminant because of its corrosive and toxic nature, and hence the production of a hydrogen fluoride-free glycolic acid for use in later processing is most beneficial, for example, where the glycolic acid may be used in the preparation of high-molecular-weight suture-grade polyglycolides suitable for human application.